1. Field of the Invention
The present invention relates to fluids having a thickened or gelled aqueous phase, e.g., water-bearing explosives of the aqueous slurry type, hydraulic fracturing fluids used in gas and oil well stimulation, and oil well drilling fluids.
2. Description of the Prior Art
The suitability of aqueous fluids for practical use often depends on the presence therein of one or more thickeners or gellants to increase viscosity, prevent fluid loss, facilitate the dispersion of solids, confer water resistance, etc. For example, in gel- or slurry-type blasting agents and explosives, which comprise inorganic oxidizing salts, fuels, and sensitizers (one or more of each of these) dissolved or dispersed in a continuous liquid, usually aqueous, phase, the entire system is thickened and made water-resistant by the addition of thickeners or gellants such as galactomannans, which swell in water or other aqueous media to form viscous colloidal solutions or dispersions commonly referred to as "sols". Crosslinking of the galactomannan with an agent such as borax, potassium dichromate, or an antimony or bismuth compound converts the sol to a firmer gel form throughout which the other phases are dispersed.
Water-bearing explosives of the type described above, when stored for extended periods, especially with exposure to elevated temperatures, are susceptible to deterioration or degradation of varying degree, as evidenced by a reduction in the viscosity of sols and a softening or reduction in the firmness of gels, or, in extreme cases, by a virtual disappearance of the sol or gel structure with a resultant separation of solid and liquid phases. The utility of a given product at any given time will depend on the extent of the degradation which it has undergone. The inhibition of noticeable deterioration over extended periods is highly desirable because a composition which tends to thin out or soften during storage, while still of possible use in the thinned or softened state as a blasting agent or explosive, is of questionable value owing to the fact that such a condition may foreshadow a more catastrophic degradation, such as liquid separation, which can occur at any time. The complete disappearance of the sol or gel structure results in a product in which the other phases are no longer uniformly dispersed and for which resistance to dilution by water in the borehole has been lost. The resulting product can be difficult and sloppy to use, and no longer reliable in performance. Limp plastic film cartridges are difficult to load into boreholes, and are prone to becoming hung up or jammed in the hole. Also it may not be possible to position a blasting cap in cartridges which have become runny or soupy, and the explosive may well be lost to the surrounding formation when the cartridges are slit open.
The stability of a slurry-type explosive under a given set of time-temperature conditions depends on many factors including the type and amount of thickener therein, the salt/water ratio, the nature of the fuel(s) and sensitizer(s) present, and whether or not the thickener is crosslinked. Greater stability is generally shown, for example, by compositions having a thickener which is present in larger amounts and/or in crosslinked form. In some cases it may be possible to improve the storage stability or shelf life of a given product, e.g., by changing the nature of the materials therein or by increasing the amount of thickener, but it may not always be feasible to make such changes from a performance and/or economic standpoint.
Instability in slurry-type blasting agents heretofore often has been attributed to the presence of particulate aluminum which may be used as a fuel and/or a sensitizer. For example, U.S. Pat. No. 3,113,059 reports that aluminum reacts exothermically with the water in the blasting agent to form hydrogen, which constitutes an explosion hazard in the oxidizing environment and, in any event, degrades the product owing to the evaporation of water therefrom. The addition of an alkali metal or ammonium phosphate, preferably diammonium hydrogen phosphate, is said to inhibit the gassing resulting from the aluminum-water reaction. U.S. Pat. No. 3,367,805 states that inhibitors such as those disclosed and claimed in U.S. Pat. No. 3,113,059 may prevent, or assist in preventing, syneresis and hence stabilize the aluminum-containing composition physically. A phosphate-type stabilizer also is used in the aluminum-containing slurries of U.S. Pat. No. 3,453,158.
Mannitol and ammonium and alkali metal phosphates are described in U.S. Pat. No. 4,207,125 as corrosion inhibitors which may be incorporated into a thickened liquid pre-mix for a slurry explosive which is to contain particulate metal.
U.S. Pat. No. 3,297,502, which discloses that the desired consistency and stability in thickened aqueous explosives often are not achieved in the presence of reactive metals, teaches the protection of metallic fuel particles with a continuous, preformed coating of an oil and an aliphatic monocarboxylic acid.
In U.S. Pat. No. 3,445,305 the aqueous solution of inorganic oxidizing salt is reported as desirably retaining an alkalinity so as to preclude corrosion of equipment and prevent the contamination of blasting agent, particularly with regard to ions such as those of iron, copper, zinc, and aluminum, which, it is stated, would inhibit or destroy a gelling system.
Urea is taught in U.S. Pat. No. 3,713,918 as retarding gas evolution from metal-sensitized, crosslinked gelled slurry explosives, and a phosphate buffer is said to be important to avoid nullification of the long-term stabilizing effect of the urea.
It is disclosed in U.S. Pat. No. 4,198,253 that guar-thickened explosive slurries containing calcium nitrate, which are said to tend to degrade more quickly at elevated temperatures than do those devoid of this salt, can be made more stable by the use of a sulfonated guar gum derivative as the thickener.
In oil field operations, water-soluble polymers such as polygalactomannan gums are employed as viscosity-increasing agents in hydraulic fracturing fluids to improve the flow characteristics and the proppant-suspending ability of the fluids (proppants are non-compressible particulate materials such as sand, walnut shells, or glass beads, which become embedded in the cracks in the formation and hold them open and porous to flow after the hydraulic pressure is released). Polymers of this type also have been used as water-loss-control agents in the wellbore fluids used in drilling operations. In drilling muds, for example, the polymer is a thickener which controls fluid loss into the formation through the wall of the well. The drilling mud carries out the cuttings of the drill bits to the ground surface, and also serves to cool the bits. After removal of the cuttings, the drilling mud is recycled to the well.
One area of concern in the use of gum-thickened aqueous fluids in these oil field operations is the possibility that the thickener will degrade owing to the high temperatures encountered in use. One suggestion for improving thermal stability has been to modify the structure of the polygalactomannan gum, e.g., according to U.S. Pat. No. 4,031,305. This patent states that sulfohydroxypropyl ethers of polygalactomannans are superior to unmodified guar gum as gelling agents in oil well fracturing compositions because they are more heat-stable under fracturing conditions. This same patent also disclosed that sodium guar-2-hydroxypropylsulfonate is superior to guar gum in heat stability and viscosity recovery properties for application as a thickener in oil well drilling muds. Methyl ethers of the polygalactomannan gums also have been described as being more heat stable than the unmodified gum (U.S. Pat. No. 4,169,798).
U.S. Pat. No. 4,025,443 reports that magnesia or magnesium hydroxide extends the stability and effectiveness of hydroxyalkylated guar gum over a higher temperature range so that it can be used as a viscosifier in clay-free wellbore fluids.